1.4 Membrane Transports

Question 1

Cellular membranes posseses two key qualities

Answer 1

They are semi-permeable (only certain materials may freely cross, large and charged substances are typically blocked)

They are selective (membrane proteins may regulate the passage of material that cannot freely cross)

Question 2

movement of materials across a biological membrane

Answer 2

may occur either actively or passively

Question 3

Passive Transport

Answer 3

Passive transport involves the movement of material along a concentration gradient (high concentration ⇒ low concentration)

Because materials are moving down a concentration gradient, it does not require the expenditure of energy

Question 4

There are three main types of passive transport:

Answer 4

Simple diffusion
Osmosis
Facilitated diffusion

Question 5

Simple diffusion

Answer 5

movement of small or lipophilic molecules (e.g. O2, CO2, etc.)

Question 6

Osmosis

Answer 6

movement of water molecules (dependent on solute concentrations)

Question 7

Facilitated diffusion

Answer 7

movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)

Question 8

Active Transport

Answer 8

involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration)

Because materials are moving against the gradient, it requires the expenditure of energy (e.g. ATP hydrolysis)

Question 9

There are two main types of active transport:

Answer 9

Primary ( direct)

Secondary (indirect)

Question 10

Primary active transport

Answer 10

Involves the direct use of metabolic energy (e.g. ATP hydrolysis) to mediate transport

Question 11

secondary active transports

Answer 11

Involves coupling the molecule with another moving along an electrochemical gradient

Question 12

Particles move across membranes by

Answer 12

simple diffusion, facilitated diffusion, osmosis and active transport

Question 13

Diffusion

Answer 13

is the net movement of molecules from a region of high concentration to a region of low concentration

Question 14

The rate of diffusion can be influenced by a number of factors, including:

Answer 14

Temperature (affects kinetic energy of particles in solution)

Molecular size (larger particles are subjected to greater resistance within a fluid medium)

Steepness of gradient (rate of diffusion will be greater with a higher concentration gradient)

Question 15

Osmosis

Answer 15

the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)

Question 16

Osm: Water is considered the universal solvent…

Answer 16

it will associate with, and dissolve, polar or charged molecules (solutes)

Question 17

Osm: Because solutes cannot cross a cell membrane unaided

Answer 17

water will move to equalise the two solutions

Question 18

Osm: At a higher solute concentration

Answer 18

there are less free water molecules in solution as water is associated with the solute

Question 19

Osmosis is essentially the diffusion of

Answer 19

free water molecules and hence occurs from regions of low solute concentration

Question 20

Osmolarity

Answer 20

is a measure of solute concentration, as defined by the number of osmoles of a solute per litre of solution (osmol/L)

Question 21

Solutions may be loosely categorised

Answer 21

as hypertonic, hypotonic or isotonic according to their relative osmolarity

Question 22

Hypertonic osmolarity

Answer 22

Solutions with a relatively higher osmolarity

Question 23

Hypotonic osmolarity

Answer 23

Solutions with a relatively lower osmolarity

low solute concentration ⇒ loses water

Question 24

Isotonic osmolarity

Answer 24

Solutions that have the same osmolarity

same solute concentration ⇒ no net water flow

Question 25

Uncontrolled osmosis will have negative effects with regards to cell viability:

Answer 25

In hypertonic solutions, water will leave the cell causing it to shrivel (crenation)
In hypotonic solutions, water will enter the cell causing it to swell and potentially burst (lysis)

Question 26

In plant tissues, the effects of uncontrolled osmosis are moderated by the presence of an inflexible cell wall

Answer 26

In hypertonic solutions, the cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape
In hypotonic solutions, the cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)

Question 27

Facilitated Diffusion

Answer 27

passive movement of molecules across the cell membrane via the aid of a mebrane protein

Question 28

carachteristics of facilitated diffusion

Answer 28

Used by molecules that are unable to freely cross the phospolipid bilayer
Process mediated by 2 types of transport proteins; channel proteins and carrier proteins

Question 29

Channel Protein

Answer 29

Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other.

Pass of substance faster, selective, contain a pore, ions from high to low along a concentration gradient

Question 30

Carrier Protein

Answer 30

Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane.

pass of substance slower, against conncentration gradient, require ATP energy.

Question 31

Vesicles move materials within cells

Answer 31

Materials destined for secretion are transported around the cell in membranous containers called vesicles

Question 32

Endoplasmic Reticulum

Answer 32

The endoplasmic reticulum is a membranous network that is responsible for synthesising secretory materials

Question 33

Golgi Apparatus

Answer 33

The vesicle is then transported to the Golgi apparatus and fuses to the internal (cis) face of the complex

Question 34

Plasma Membrane

Answer 34

Vesicles containing materials destined for extracellular use will be transported to the plasma membrane

The vesicle will fuse with the cell membrane and its materials will be expelled into the extracellular fluid

Question 35

Bulk Transport

Answer 35

-The membrane is principally held together by weak hydrophobic associations between the fatty acid tails of phospholipids that allows fluidity and flexibility, as the phospholipids can move around to some extent

This allows for the spontaneous breaking and reforming of the bilayer, allowing larger materials to enter or leave the cell without having to cross the membrane (this is an active process and requires ATP hydrolysis)

Question 36

Endocytosis

Answer 36

The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane

An invagination of the membrane forms a flask-like depression which envelopes the extracellular material
The invagination is then sealed off to form an intracellular vesicle containing the material

Question 37

There are two main types of endocytosis:

Answer 37

Phagocytosis: The process by which solid substances are ingested (usually to be transported to the lysosome)

Pinocytosis: The process by which liquids / dissolved substances are ingested (allows faster entry than via protein channels)

Question 38

Exocytosis

Answer 38

The process by which large substances exit the cell without crossing the membrane

Vesicles (typically derived from the Golgi) fuse with the plasma membrane, expelling their contents into the extracellular environment